Currently, titanium oxide is often used as a photocatalyst material used for purposes such as antifouling and deodorization. In addition, photocatalyst materials are used in various areas including construction materials for outdoor use, construction materials for indoor use, and materials for household electrical appliances such as lighting equipment, refrigerators, air conditioners and toilet equipment. However, as titanium oxide is excited in the ultraviolet range, sufficient photocatalytic performance cannot be obtained in indoor areas which receive a smaller amount of ultraviolet light.
Therefore, research and development of photocatalysts having photocatalytic performance even under visible light, so-called visible light photocatalysts, has been carried out recently.
As a visible light photocatalyst, tungsten oxide is known. A technique for using tungsten oxide as a visible light photocatalyst is disclosed, for example, in Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2001-152130).
With regard to the tungsten oxide as a visible light photocatalyst disclosed in Patent Document 1, a tungsten oxide film is formed on a substrate by sputtering process, and the crystal structure of the tungsten oxide is predominantly triclinic. However, the sputtering process involves exposure of substrates to high temperature. Therefore, the sputtering process cannot be applied in some cases, depending on the heat resistance temperatures of substrates. Further, sputtering operations are often performed in high-vacuum chambers or the like, and involve complicated vacuum management and processes. Therefore, there has arisen a problem that the production cost may be extremely high depending on the shapes and sizes of substrates, and a problem that it is difficult to form a film over a wide area, for example on a construction material.
On the other hand, use of tungsten oxide powder as a photocatalyst is also considered. If tungsten oxide is in powdered form, a film can be formed by mixing the powder with an organic binder such as resin and applying the mixture onto a substrate. Therefore, there is no need to expose substrates to a high temperature, and it is possible to form films on materials having a wide area such as construction materials.
As a method for producing a tungsten oxide powder, for example, there is a method for obtaining a tungsten trioxide powder by heating ammonium paratungstate (APT) in air, as described in Japanese Unexamined Patent Application Publication No. 2002-293544 (Patent Document 2). In the proposed method, a triclinic tungsten trioxide powder having a particle size of 0.010 μm (BET specific surface area is 82 m2/g) is obtained by the method wherein APT is heated in air. However, the manufacturing efficiency is low because 20 hours or longer of kneading is required to stabilize the BET specific surface area.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-152130    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-293544    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2006-102737    Non-Patent Document 1: Journal of Solid State Chemistry, Vol. 143, No. 24, p. 32, 1999